Organically-Templated Lanthanide Fluoride Frameworks,

The crystal structure of the parent phase [C2H10N2]0.5[Y2F7] (YF-1) has been reported 6.

Exploration of the suitability of crystal structure (YF-1) towards the other lanthanides is one of our key objectives. Lanthanide analogues (Nd3+– Lu3+) of this structure type framework have been effectively synthesised (Table 4.03). This is the first organically templated trivalent lanthanide fluoride framework. The crystal structures of [C2H10N2]0.5[Yb2F7] and

[C2H10N2]0.5[Lu2F7] were studied by single crystal X-ray diffraction and the other phases were

characterised by Rietveld refinement and powder X-ray diffraction analysis. Meantime, La3+, Ce3+, and Pr3+analogues of YF-1 phase have not been effectively synthesised in this method.

Lanthanide Product Morphology

Y [C2N2H10]0.5[Y2F7] Single Crystal4 La LaF3 Powder Ce CeF3 Powder Pr PrF3 Powder Nd [C2N2H10]0.5[Nd2F7] Powder Tb [C2N2H10]0.5[Tb2F7] Powder

Dy [C2N2H10]0.5[Dy2F7] Powder+ Single Crystal Ho [C2N2H10]0.5[Ho2F7] Powder

Er [C2N2H10]0.5[Er2F7] Powder Yb [C2N2H10]0.5[Yb2F7] Single Crystal

Lu [C2N2H10]0.5[Lu2F7] Single Crystal

*

Powder XRD patterns for the Eu and Gd phases could not be obtained due to prohibitive fluorescence effects.

4.03.1.1 Single Crystal X-ray Analysis of [C2H10N2]0.5[Ln2F7] (Ln = Yb and Lu)

Single crystals of the Lu and Yb derivatives of YF-1 have been prepared, and crystallographic details of those derivatives are shown in Table 4.04. Bond lengths and full details of refinements of both Lu and Yb derivatives are presented in Table 4.05, 4.06, 4.07and 4.08.

The crystal structures of [C2H10N2]0.5[Yb2F7] and [C2H10N2]0.5[Lu2F7] confirm that the

[C2H10N2]0.5[Y2F7] type structure is adapted for smaller cations within the lanthanide series.

Formula [C2N2H10]0.5[Yb2F7] [C2N2H10]0.5[Lu2F7]

Fw 510.14 513.95

Space Group Fddd (orthorhombic) Fddd (orthorhombic)

a [Å] 7.9565(11) 7.958(3) b [Å] 13.906(2) 13.894(3) c [Å] 22.415(4) 22.410(2) V [Å3] 2480.1(7) 2477.8(11) Z 16 16 T/K 113 ρcalc[g cm-3] 5.465 5.542 μ[mm-1] 30.06 31.94 Crystal Size [mm] 0.3 x 0.3 x 0.3 0.06 x 0.1 x 0.1 F(000) 3536 3568 Reflns Collected 4125 6322 Independent Reflns 706 1000 Rint 0.022 0.0893

Obsd Data [I>2σ(I)] 584 781

Data/Restraints/Parameters 706/0/51 1000/0/781

GOF on F2 1.04 1.933

R1,wR2(I>2σ(I)) 0.019, 0.046 0.063, 0.164

R1,wR2(all data) 0.025, 0.044 0.072, 0.158

Atom site x y z Uiso/Ueq/ Å2 Yb(1) 16f 0.125 0.44296(2) 0.125 0.00291(11) Yb(2) 16g 0.125 0.625 -0.000282(13) 0.00393(11) F(1) 16g 0.375 0.375 0.12729(17) 0.0065(9) F(2) 32h 0.2607(4) 0.54193(19) 0.06674(11) 0.0089(6) F(3) 32h 0.1322(3) 0.3490(2) 0.20562(13) 0.0073(6) F(4) 32h 0.3038(4) 0.5132(2) 0.19874(12) 0.0084(6) C(1) 32h 0.1289(15) 0.6387(9) 0.5418(3) 0.0094(19)* N(1) 32h 0.1256(14) 0.6692(9) 0.6049(5) 0.020(3)*

*Isotropic only (disordered sites, 50% occupancy)

Table 4.05:- Final Refined Atomic Parameters for [C2N2H10]0.5[Yb2F7]

Atom site x y z Uiso/Ueq/ Å2

Lu(1) 16f 0.125 0.44252(4) 0.125 0.0085(3) Lu(2) 16g 0.125 0.625 -0.00036(3) 0.0096(3) F(1) 16g 0.375 0.375 0.1259(4) 0.005(2) F(2) 32h 0.2567(13) 0.5426(6) 0.0668(4) 0.0159(19) F(3) 32h 0.1335(10) 0.3484(7) 0.2059(4) 0.0100(17) F(4) 32h 0.3055(11) 0.5122(6) 0.1993(4) 0.0091(15) C(1) 32h 0.123(5) 0.674(3) 0.6058(15) 0.023(8)* N(1) 32h 0.123(5) 0.638(3) 0.5402(10) 0.018(6)*

*Isotropic only (disordered sites, 50% occupancy)

Table 4.07:- Bond Lengths of [C2N2H10]0.5[Yb2F7] Bond Distance/Å Lu1 - F1 x 2 2.1956(8) Lu1 - F2 x 2 2.171(2) Lu1 - F3 x 2 2.232(9) Lu1 - F4 x 2 2.398(8) Lu2 - F1 x 1 2.767(11) Lu2 - F2 x 2 2.158(9) Lu2 - F3 x 2 2.184(8) Lu2 - F4 x 2 2.280(8) Lu2 - F4 x 2 2.410(7)

Table 4.08:- Bond Lengths of [C2N2H10]0.5[Lu2F7]

The structure of [C2H10N2]0.5[Yb2F7] consists of two different crystallographic sites of

ytterbium centred polyhedra, which are YbF8 square antiprisms and YbF9 mono-capped

square antiprisms. The size of the two crystallographically different ytterbium (Yb3+) polyhedra in [C2H10N2]0.5[Yb2F7] is smaller than that of yttrium (Y3+) in [C2H10N2]0.5[Y2F7]

which corresponds to Ln3+ ionic radii difference (r(Yb3+) = 0.985 Å, r(Y3+) = 1.019 Å) 8.

Bond Distance/Å Yb1 - F1 x 2 2.2028(3) Yb1 - F2 x 2 2.183(3) Yb1 - F3 x 2 2.231(3) Yb1 - F4 x 2 2.389(3) Yb2 - F1 x 1 2.744(4) Yb2 - F2 x 2 2.181(3) Yb2 - F3 x 2 2.200(2) Yb2 - F4 x 2 2.307(3) Yb2 - F4 x 2 2.403(3)

These independent Yb/F polyhedra are linked together via shared F atoms to form a three- dimensional framework structure with ethylenediammonium cations in cavities. The organic cations are 50:50 disordered around a high symmetry (point symmetry 222) position. The crystal structure can be seen in Figure 4.03 which shows the 3-D framework without the organic template, along the [011] direction. Figure 4.04shows the full structure alongaaxis.

Figure 4.03:- Crystal Structure of [C2H10N2]0.5[Yb2F7] viewed down [011]. ([C2H10N2]2+

moiety is not shown)

The parent phase of this structure, [C2H10N2]0.5[Y2F7], was the first organically templated

yttrium fluoride. The high co-ordination number of Y and a large number of symmetry operations lead to a complicated structure, as follows (Figure 4.05).

Figure 4.05:- Coordination Environments of Two Crystallographically Distinct Yb Sites in [C2H10N2]0.5[Yb2F7]

This structure can be described in terms of simple motifs. The two different Yb3+ sites produce a 4-membered ring window with two edge sharing YbF8 square antiprisms and two

edge sharing YbF9 mono-capped square antiprisms. The organic cation, disordered

ethylenediammonium,[C2H10N2]2+, sits behind the 4-membered ring window (Figure 4.06).

The YbF8square antiprisms form a large helical chain along the a-axis which is connected by cis corners of the YbF8 square antiprisms. A single YbF8 square antiprism face sharing with

YbF9 mono-capped square antiprism creates a helical chain by symmetry operation of 2-fold

screw axis. YbF9 mono-capped square antiprisms form another chain along the [010]

direction sharing an edge with one another (Figure 4.07).

Figure 4.07:- YbF9Edge Sharing Chain along the [010] Direction

4.03.1.2 Crystal Structures of Lanthanide Family of [C2H10N2]0.5[Y2F7],

[C2H10N2]0.5[Ln2F7] (Ln= Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho and Er)

Initial confirmation of the isomorphous lanthanide series has been provided by powder X-ray diffraction. Lanthanides of Ce, Pr, Nd, Tb, Dy, Ho and Er form powder samples with the YF- 1 structure. High quality X-ray patterns of the Eu and Gd analogues could not be obtained due to the prohibitive fluorescence effects. Rietveld refinements were carried out for the Nd, Tb, Ho, Er, Yb and Lu phases, using the [C2H10N2]0.5[Y2F7] structural model (Table 4.04 and

Table 4.05). The refinements are carried out by varying only the lattice parameters and profile parameters (peak shape, background and detector zero-point). These refinements were adequate to derive precise lattice parameter values. Rietveld refinements of [C2H10N2]0.5[Er2F7] and [C2H10N2]0.5[Lu2F7] will be presented as follows (Figure 4.08 and

Figure 4.08:- The Rietveld Profile of [C2H10N2]0.5[Lu2F7]. Observed Data Red, Calculated

Profile Green and Difference Profile Purple

Figure 4.09:- The Rietveld Profile of [C2H10N2]0.5[Er2F7]. Observed Data Red, Calculated

Qualitative analyses of SEM/EDX measurements have been carried out and confirm substitution of the lanthanide in these compounds. The following examples show the qualitative characterisation for these materials (Figure 4.10, Figure 4.11andFigure 4.12).

Figure 4.10:- SEM Image of [C2H10N2]0.5[Yb2F7]

Figure 4.12:- SEM/EDX of [C2H10N2]0.5[Yb2F7]

Crystallographic trends are determined by calculating the unit cell volume of lanthanide framework versus Ln3+radii. A linear relationship of decreasing unit cell volume with respect to decreasing ionic radius (eight co-ordinated) from Tb3+ to Lu3+ is observed. However, the volume for the [C2H10N2]0.5[Nd2F7] clearly illustrates a deviation from the linear relationship

(Table 4.09, Figure 4.14). (Figure 4.13 shows that example of qualitative analysis of SEM/EDX for the confirmation of the lanthanide analogue of YF-1 phase)

The lanthanide contraction decreases the ionic radii with increasing atomic number of lanthanide series. Therefore, the decreasing of unit cell volume from Tb3+ to Lu3+ is as expected. However, the deviation of the Nd3+ analogue suggest a limitation of the YF-1 phase stability due to lattice strain at this composition.

Figure 4.13: Quantitative Analysis of SEM/EDX for the Confirmation of Lanthanide Analogue of YF-1 Framework. (a) [C2H10N2]0.5[Er2F7] and (b) [C2H10N2]0.5[Nd2F7]

Lanthanide a / Å b/ Å c/ Å Unit Cell

Volume/ Å3 Lu* 7.958(3) 13.894(3) 22.410(2) 2477.8(11) Yb* 7.9839(4) 13.9299(7) 22.4415(11) 2495.9(3) Er$ 8.0337(3) 14.0559(6) 22.596(1) 2551.5(2) Ho$ 8.0577(4) 14.1293(7) 22.6912(11) 2583.4(3) Tb$ 8.0879(10) 14.2345(16) 22.890(3) 2635.2(6) Nd$ 8.0891(5) 14.2360(8) 22.8894(13) 2635.9(3)

Table 4.09: Lattice Parameters for the [C2H10N2]0.5[Nd2F7] series from Single Crystal Data*

Figure 4.14:Unit Cell Volume of the Pure Ln3+ Analogues of YF-1 versus Eight-Coordinate Ionic Radius.